[time-nuts] Temp/Humidity control systems?

Neville Michie namichie at gmail.com
Thu Oct 27 20:46:02 EDT 2016

Dew Point measurement technology is limited by surface energy problems,
in the near vicinity of the surface,Van der Waals or London forces create uncertainties
in the physical processes and dew point instruments always seem to have these uncertainties.
The physical equilibrium on the surface of a water film has proved reliable, hence the reliance 
of reference instruments using psychrometery. Dew point technology is mainly used for measurement
at extremes of dewpoint well outside the conditions in ambient weather and in in-line situations 
in industrial control where precision gives way to convenience.

Neville Michie

> On 28 Oct 2016, at 11:05 AM, Poul-Henning Kamp <phk at phk.freebsd.dk> wrote:
> --------
> In message <20161027134312.GA18158 at panix.com>, Ron Bean writes:
>> And since this is time-nuts: Measuring humidity accurately is tricky. 
>> According to people who have tested them, commercial electronic humidity 
>> sensors, when tested in a lab, have never come anywhere close to the 
>> accuracy claimed in the data sheet.
> The main problem in measuring humidity is physical gradients:  It is
> incredibly hard to create a volume of homogenous humidity on a planet
> which has gravity, and for that reason, a lot of labs are not anywhere
> near as accurate as they think they are.
>> The exception is the "cold mirror" type of sensor, which measures the 
>> dewpoint by cooling a mirror and bouncing a light off it to sense the 
>> temperature where dew condenses on it. Those are expensive, and they 
>> require maintenance to keep the mirror clean.
> And they are comparatively slow, last I saw one it could only do
> a measurement every second minute.
> -- 
> Poul-Henning Kamp       | UNIX since Zilog Zeus 3.20
> phk at FreeBSD.ORG         | TCP/IP since RFC 956
> FreeBSD committer       | BSD since 4.3-tahoe    
> Never attribute to malice what can adequately be explained by incompetence.
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